The concept of dual-acting solenoid actuators, particularly for engine valve actuation, goes back to the early 1900s. The historic approach is illustrated schematically in FIG. 1 (Prior Art), wherein an armature 120 drives a shaft 130 (labeled at both top and bottom ends), which may typically be coupled to a cylinder valve (not shown) for operation of a camless internal combustion engine. The armature and shaft are restored by one or more springs (not shown) toward a position intermediate between upper magnetic yoke 100 and lower magnetic yoke 105. Yoke 100 is driven electrically by coil 110, whose wire leads (not shown) are energized by an electronic driver circuit (not shown). Yoke 105 is similarly driven similarly by coil 115, whose wire leads (not shown) are energized by a second electronic driver circuit (not shown). When a driver circuit causes an electric current to flow through coil 110, then a magnetic field is induced in yoke 100, with part of this field bridging across an air gap to armature 120, which is thereby attracted upward toward 100. Similarly, when a second driver circuit causes a current to flow in coil 115, a magnetic field is induced in yoke 105, attracting armature 120 downward toward 105. Using appropriate electrical output signals from the two electronic drivers, it is possible to move armature 120 into either of two latching positions, on the upper side against yoke 100 or on the lower side against yoke 105.
Variations on the above approach to hardware design and actuation control are possible. The armature and two yokes might, for example, be configured as a circular or truncated-circular armature attracted to yokes having the general form of pot cores. Alternatively, the armature might be rectangular and might be drawn alternately to opposite E-core yokes. In yet another configuration, a horizontal armature might rock up and down about a rotary shaft at a lateral end of the armature between an over-and-under pair of electromagnetic yokes. These configurations share in common that there are two electromagnetic yokes and two windings, independently driven by two electronic driver circuits.
Since the present invention concerns improvements in the electronics to drive an otherwise “conventional” dual-acting and dual-winding solenoid, it is worth discussing some of the constraints on achieving effective and efficient solenoid actuation. Each of the two windings (110 and 115) of FIG. 1 (or in a variation on the topology of FIG. 1) needs to be driven strongly in both “forward” and “reverse” voltage polarities: “forward” to build up the magnetic field quickly and overcome resistance losses at high peak currents for armature pull-in; and “reverse” to reduce the forward current and attenuate the magnetic field quickly for armature release. If an active reverse voltage is not available (for example, if only passive resistance and one or more forward diode voltage drops is available to slow the forward flow of electric current), then the armature release will be slowed substantially by an un-attenuated magnetic field as the armature pulls away from the releasing yoke. The attracting field at pull-away will oppose the force of the spring that accelerates the armature toward the opposite yoke, thus removing mechanical energy that is likely to be needed for getting the armature within pull-in range of the opposite yoke. Furthermore, in order to build up and attenuate the magnetic field rapidly with a given limited power supply voltage, the number of turns in each winding (such as 110 or 115) is strictly limited—the slew rate for changing magnetic flux linkage varies inversely as the number of turns for a given drive voltage. When the winding count is thus set low enough to achieve the needed magnetic slew rate, then the total resistance in each winding is a small fraction of an ohm, while the electric current needed for magnetic pull-in toward latching is typically measured in tens of amperes (for example, in a 12-volt or 42-volt automotive system.) To minimize electrical losses at the necessary high currents and low impedances, therefore, the electronic drive circuitry is generally a Pulse Width Modulation (PWM) circuit employing output devices with very low on-resistances (as with Field Effect Transistors or FETs) or very low forward voltage drops (as with bipolar transistors or related devices). The solution to this electronic drive design problem, with single-supply operation, is commonly to employ a full-wave bridge circuit with an active pull-up and pull-down device for each of the two winding leads on each of two coils: a total of four leads and eight high-current driver devices. For purposes of discussion, a driver circuit capable of applying active forward and reverse voltages to a single winding will be referred to as a “single driver,” which might consist of a full-wave bridge using a single supply or a totem pole topology with dual positive and negative power supplies sharing a common ground or current return path. In this context, a conventional dual-acting solenoid system requires a “dual driver” consisting of a pair of single drivers.
Two “single-driver” approaches for dual-acting solenoids have previously been described for reducing the wiring and electronic hardware needed to operate a dual acting solenoid actuator. First, in the system of European Patent EP0992658 and U.S. Pat. No. 6,651,954 B1, Porcher et. al. describe a simplified system achieving solenoid action of a single armature with latching in either of two positions. As shown in FIG. 2 (Prior Art, adapted from U.S. Pat. No. 6,651,954), a single winding 38 creates a magnetic potential difference between yoke element 36 on the left and mirror-image yoke element 37 on the right. Each of two curving jaws 36 and 37 of the yoke carries a magnetic polarity, one jaw at north polarity and the other at south. Each of the jaws meets one end of a moving armature 22 in either of two axial latching positions. When the armature is far off-center near one of these latching positions, magnetic forces predominate across the smaller yoke-armature gap on the side close to latching, giving rise to a strong force toward completed closure and latching on that side. Thus, application of current to the single winding can be used to latch the armature in either of two positions. Some drawbacks to this invention are noted here. The geometric constraints of bringing magnetic flux down from a winding on the top end of the solenoid to a bottom latching area result in a substantial increase in the vertically-projected footprint area of the solenoid, as compared to conventional solenoids with separate windings on separate yokes. Space is required for the flux-carrying cross-section to bring flux down to the bottom latching poleface area. Further space is required to provide an adequate lateral gap between the sides of the armature and the adjacent inside vertical surfaces of the yoke. Narrowing the lateral gaps between armature and yoke causes high leakage of flux across the armature for all axial positions in the armature travel, resulting in flux that creates no axial attraction for moving the solenoid armature along its intended travel axis. This non-functional leakage flux uses flux-carrying capacity in both the armature and the yoke, lowering the achievable magnetic forces as limited by saturation of the yoke. The non-functional flux also results in a high stray winding inductance, which must be overcome by higher drive voltages.
The second previous approach for reduced wiring and switching hardware, described by the present inventors (Bergstrom and Seale) in U.S. Pat. No. 6,724,606, is to maintain a relatively conventional dual solenoid magnetic topology but simply to wire the two yokes in series. As illustrated in FIG. 3 (Prior Art), winding area 310 is associated with upper E-core yoke 300 while winding area 315 is associated with lower E-core yoke 305, but the two winding areas 310 and 315 are interconnected via wire 365, forming a single electrical circuit between terminals 360 and 370. Other features are similar to the prior art configuration of FIG. 1, for example vertical shaft 330 of FIG. 3 corresponding to shaft 130 of FIG. 1 and armature 320 corresponding to 120.
Functionally speaking, series interconnection is not a bad tradeoff when the armature is not too close to its center position. For an off-center armature, most of the impedance and over half the electrical losses are associated with the “working” side of the series-connected yokes—the side closer to the armature. On this “working” side there is a higher inductance, higher flux levels, and consequently higher magnetic hysteresis losses. The yoke farther from the armature adds its share of resistive loss at all times, but as explained in U.S. Pat. No. 6,724,606, winding resistances in typical valve actuation solenoids are not the most important sources of energy loss. When wound with few enough turns to permit a needed flux slew rate (as discussed above), winding resistance is typically only a small fraction of an ohm. Thus, non-winding circuit resistances in electronic switching devices, circuit board traces, and connectors tend to predominate over winding resistances, unless there is a significant monetary investment in large electronic components and large or thick board traces. In a single-winding configuration, one set of driver electronics is used instead of two. Part of the electronic cost saving can therefore go into larger switching devices, larger or thicker foils, etc., offsetting part of the resistance increase of the series windings while the overall system cost is still reduced.
Both the parallel magnetic topology of FIG. 2 and the series winding topology of FIG. 3 present startup problems—magnetic purchase to get started is very low unless there is a considerable magnetic asymmetry at the spring-neutral rest position. FIGS. 2 and 3 both indicate ways of creating magnetic asymmetry for a centered armature. In FIG. 2, magnetic element 84 creates this asymmetry, being attracted upward when the armature is centered and a winding current is applied. In FIG. 3, the armature is made asymmetric by beveling surface 325 near the outer edge of armature 320 and providing a sloped matching surface on yoke 305. There may be reasons, however, for biasing the armature-restoring springs to give the entire armature an off-center spring-neutral position. Note, for example, that armature 320 is shown in a spring-neutral position that is off-center below the midpoint between upper and lower latching positions. A certain asymmetry might be called for in optimizing a valve actuator for an asymmetric mechanical load. For example, an exhaust valve actuator can benefit from a spring that is biased to favor opening of the valve more than closing, since valve-opening must be performed against the opposing pressure of exhaust gases.
Even with asymmetries of armature construction and centering, single-driver dual-latching solenoids are likely to have very little starting force. Even in conventional topologies (as in FIG. 1) with separate drivers on each winding, the force of attraction between the centered armature and either yoke tends to be low. The achievable magnetic pull increases steeply in the final small fraction of travel from centered to latching position. Thus, it is commonly required to alternately energize the upper and lower winding circuits at a mechanical resonance of the armature and its restoring spring system, building up oscillatory amplitude until the armature comes close enough to be pulled in and latched by a yoke. Once latching is achieved on either side, the single-driver approach is comparatively more effective. The starting problem described here is addressed by the invention disclosed below.
Another area of concern for the present invention is sensorless determination of armature position and velocity, particularly for use in dynamic servo control of armature motion. An important application for effective servo control is the soft landing of engine valves, to reduce noise and extend valve and actuator life. An apparatus and method for sensorless determination of armature position, including for servo control, has been described by an author of the present patent (Bergstrom) in U.S. Pat. No. 6,249,418. In the case of a dual-acting solenoid, Bergstrom's invention would use information from a single solenoid winding (for example, recent history of measured current and the known sequence of applied voltages) to determine the effective magnetic gap between the armature and the magnetic yoke on one side. The technique might be applied to both yokes of a dual-acting solenoid, so that position would be determined redundantly or based on the one of two yokes that yields better information about position at a given moment. When the four solenoid wires are interconnected to bring out fewer wires, for example three, then the problem of sensorless determination of position or velocity is altered and problems arise. As will be seen, the present invention addresses this sensorless control issue.